Prescaler circuits for use in high speed dividers, frequency synthesizers and the like are well known in the art. A dual modulus prescaler is a counter whose division ratio or modulus can be switched from one value to another by an external control signal. Commonly known implementations of prescalers utilize a counter circuit consisting of series-coupled flip flop circuits which are used to obtain a fractional output signal from a clock signal. Thus, a prescaler can divide by a first factor when the external control signal has a first state or by a second factor when the external control signal has a second state.
Current and future telecommunication devices require very high speed prescaler circuits. For example, cellular radiotelephones that operated in the 800-900 MHz band are being joined in the market by Personal Communication Systems (PCS) and satellite radiotelephones operating at 1800-2000 MHz. A prescaler for use in such radiotelephones must operate at these high frequencies under worst case conditions.
In addition, modern telecommunication devices have placed additional operational requirements on the performance of prescalers in these devices. Portable radiotelephones require very low power dissipation to enhance portability and long-term use between battery recharge. Thus, prescalers used in portable devices must have very low power dissipation. In addition, to reduce power dissipation even more, supply voltage levels for electronic circuits in portable devices are being lowered. Typical values for future operating voltages are 2.0 volts with 1.7 volts as a worst case value. A suitable prescaler must operate at very high frequencies and at very low voltage while dissipating little power.
Accordingly, there is a need in the art for a prescaler circuit providing high speed operation at low power levels and a low operating voltage.